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Demons are a curious lot.  Demons are the servants and other residents of Hell.  They can possess as well as ghosts can, but their toll on the body is much different.  If you’ve ever seen a zombie movie, it becomes about the same.  The person they possess loses all sense of themselves.  They show decomposition the longer they are possessed, their voices become darker and more sinister.  The longer a person is possessed, the more of themselves they lose.

If a demon is inside them long enough, that demon will eventually claim that person and they die, thus the demon is now free to keep that form and walk around like a mortal person.  The decomposition goes away because the soul that was fighting inside is now gone, the voice of the person as well as the memories stay with the demon, like they never existed at all.  Research and speculation has found that if the demon keeps the body and the soul leaves, the soul will either die, or be trapped forever on another plane of existance.

If the possession is caught in time and an exorcism is performed, there is an 85% chance that the person can be save.  That is only speculation, there is no way of knowing for sure.  If the demon ends up keeping the body, Hell has yet another foothold on earth, thus leading us to Hell on earth.